`T | None` does not work when T is an union

[Hakuyume]

Bug Report

mypy can not handle T | None when T is also an union.

To Reproduce

Run mypy on the following code.

def unwrap[T](value: T | None) -> T:
    assert value is not None
    return value


def f(value: int | str) -> None:
    pass


def main() -> None:
    value: int | str | None = 42
    f(unwrap(value))

https://mypy-play.net/?mypy=latest&python=3.12&gist=7c62868a7c9a0afc4853487fdd605f4b

Expected Behavior

mypy does not show any errors.
unwrap(value) converts int | str | None to int | str.

Actual Behavior

main.py:12: error: Argument 1 to "f" has incompatible type "object"; expected "int | str"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

mypy handles unwrap(value) as object and raises an error.

Your Environment

• Mypy version used: 1.18.2
• Mypy command-line flags: default of playground
• Mypy configuration options from mypy.ini (and other config files): default of playground
• Python version used: 3.12

[A5rocks]

This is yet another case where mypy using join is confusing (compared to using a union). Probably it would make sense to switch this? (I think this is the case?)

As a workaround, you can rely on type context. I think this should work?

x: int | str = unwrap(value)

(If it doesn't then that's a bug IMO and should get its own issue)

---

Actually maybe mypy already does use type context for the original code snippet (I can't remember the rules...). Something that definitely doesn't is:

x = unwrap(value)
f(x)  # should not error

[Hakuyume]

@A5rocks Thank you for sharing workaround. I tried the following code but it does not work.

def unwrap[T](value: T | None) -> T:
    assert value is not None
    return value


def f(value: int | str) -> None:
    pass


def main() -> None:
    value: int | str | None = 42
    x: int | str = unwrap(value)
    f(x)

https://mypy-play.net/?mypy=latest&python=3.12&gist=26813015e91e6cfb3fc0e1fbd2452e4c

main.py:12: error: Incompatible types in assignment (expression has type "object", variable has type "int | str")  [assignment]
Found 1 error in 1 file (checked 1 source file)

[ilevkivskyi]

I think this is a duplicate of #17383, let's continue discussion there.